All things in the world are either living or nonliving. Squirrels are living creatures, but stones are not. Trees are living things, but tennis balls are not. Birds are alive, but beds are not. Humans are living beings, but houses are not.

The line between the living and the nonliving is not always so clear, however. The differences are not always easy to see even though all living things have certain characteristics in common. From big to small, from giant whales to tiny bacteria, all living creatures are alike in the following basic ways. They can

take in food and produce energy

grow

move

respond to their environment

adapt to their environment

reproduce

These traits, taken all together, separate living things from nonliving things.Characteristics of Living Things

All living things are called organisms. Many organisms, such as animals, grass, and trees, are familiar — we see them every day. But not all organisms are visible to the naked eye. Some can only be seen through a microscope. These tiny one-celled organisms are called microbes,or micro-organisms.

Taking in Food and Producing Energy

All organisms must either make their own food or take in food. Green plants make their own food through the process of photosynthesis. Animals must take in their food. Once it is in the body, food is broken down into simpler substances. Some of these substances are used to make the organism larger or to repair cells. Some combine with oxygen to produce energy. The process in which food is broken down into simpler substances and combined with oxygen to provide energy and materials for grow this called metabolism.

When wood burns, it combines very quickly with oxygen and releases a great deal of heat energy. In cells, the oxygen combines slowly with substance such as fats and carbohydrates and releases a limited amount of heat energy. This is called respiration. Respiration is more than the simple act of breathing in and out. It refers to the entire process of taking in oxygen, using the oxygen in the body, and then breathing out the waste product, which is the gas carbon dioxide.

Growth

All living organisms grow and develop. Growth and development are the processes by which a fertilized egg becomes a tree, an elephant, or a newborn human baby. Nonliving objects such as tables, tissue paper, and tennis balls do not grow and change by themselves.

In a single-celled organism all of the activities needed to stay alive are carried out by the single cell. When the cell divides, the two new cells formed are smaller than the original parent cell. The new cells then grow until they are the same size as the parent.

Each many-celled organism starts as a single cell. The cell divides until it becomes many cells. These cells specialize. In animals, some become the skin, others become the bones, and still others become eyes, ears, and noses. In plants, different cells become stems, leaves, and flowers.

All organisms live for a certain period of time and then they die. The length of time from birth to death is known as the life span. During that time each organism passes through four stages in its life cycle — birth, growth, maturity, and death.

Movement

Some of the energy produced by the process of respiration is changed into motion or movement. For example, the material inside a cell is always moving. In addition to the movement within the cell, some one-celled organisms that live in water are able to move about by means of whip like extensions called flagella. Others have many hair like cilia that allow them to swim through the water. In many-celled animals, special cells called muscle cells are responsible for movement. These cells have the ability to contract, or become slightly shorter. When each one of the millions of muscle cells in an animal's legs contracts a tiny bit, the leg moves and the animal is able to walk or run.

Organisms such as plants and fungi cannot move about, but they can still become widely scattered. Their seeds and spores, which are tiny reproductive cells, can travel long distances. They may be light enough to be carried by the wind or to float on water. Some stick to passing animals, insects, or birds and travel far from the parent.

Responsiveness

Responsiveness, or the ability to react to conditions or events on the outside, is an important characteristic of living things. For example, simple organisms with only one cell can respond to light or food by moving closer to the food and avoiding the light. When certain fish swim near the tentacles that hang from the rim of a jellyfish's bell-shaped body, the jellyfish immediately responds. It injects a paralyzing substance into the fish's body. Then the jellyfish traps the helpless fish in its sticky tentacles and eats it.

Plants respond to light, to gravity, and to the presence of water. Some plants, such as those that trap insects in specialized leaves, respond to touch. The leaves of a houseplant placed on a windowsill will turn to face the sunlight; the growing tip of the stem always turns upward. At the same time, the roots of the plant turn downward and toward a source of moisture. The slow movements of plants in response to stimuli are called tropisms.

Adaptation

When organisms fit in very well with their environment, they are said to be well adapted. For example, the cactus is a plant that is well suited to live where there is little water. Over time the leaves of cactus plants modified, or changed, into needlelike spines. Much less water is lost from this type of leaf than from a broad, thin leaf. Because the leaves have been reduced to spines, the important process of photosynthesis is carried out in the thick green stems of the cactus. The stems are coated with a wax like covering that also cuts down water loss. In fact, the plant stores water in its thick, fleshy stems. When the stems of a barrel cactus are mashed, they produce several quarts of watery liquid. All of these adaptations help the cactus survive in the extreme heat and dry conditions of a desert. Polar bears live in cold climates. They have a thick layer of fat under their white fur that keeps them warm.

Charles Darwin (1809-82) put forth a theory to explain how living things adapt to their environment. Darwin pointed out that there is variation within every group of plants or animals. Some are taller, some shorter. Some are stronger, some weaker. Some need high temperatures, others can live with less heat.

Since there is never enough food and space to support every organism that is born, individuals have to compete in order to survive. Those that cannot compete successfully for food and spaces may die. If they die very young, they leave no offspring.

The organisms that have adapted to their environment do well. They are healthier and live longer. They leave more offspring, and their offspring resemble them. The traits that helped them survive are passed from generation to generation.

An example of this theory is the giraffe. Several theories have been proposed to explain why its long neck is a favorable characteristic. One theory states that long ago the ancestors of the giraffe had rather short necks. The giraffes with the shortest necks could reach only the lowest branches of the trees. Since there were many giraffes, those leaves were soon gone and many short-necked giraffes died of starvation.

However, among the giraffes that lived long ago were some with slightly longer necks. These giraffes were able to reach higher branches and get more food. They lived longer and had more offspring. Their offspring tended to be taller than the other giraffes. After many generations, giraffes came to have long necks. Long-necked giraffes are very well adapted to life on the African plains.

Reproduction

Living things reproduce themselves. The robin flying across the sky hatched from an egg laid by another robin. The oak tree growing in the forest came from a seed dropped by another oak tree. Each of us began with the joining of cells from two other human beings.

Reproduction is the ability of any organism to produce new organisms similar to the parents. Most plants and animals reproduce through the union of two specialized cells. This is known as sexual reproduction. The female parent produces an ovum, or egg cell. The male parent produces a sperm cell. The union of these two cells is called fertilization.

Some simple one-celled organisms reproduce in a different way. The amoeba and paramecium are two such organisms. In these organisms, the single cell simply divides in two and produces two new cells that are just like the parent. This process is called asexual reproduction.

The Chemistry of Life

Matter is made up of elements, either singly or in various combinations. An element is a substance that cannot be broken down into a simpler substance by ordinary means.

Every element is made up of units called atoms. The air you breathe is made up of atoms of oxygen and nitrogen, and the iron in your red blood cells is made up of atoms of iron.

Atoms are almost always joined with other atoms to form molecules. A molecule is made up of two or more atoms. Some molecules are composed only of atoms of one element. For example, a molecule of oxygen (O2) contains two atoms of oxygen. But an atom may also combine with one or more atoms of different elements. Substances formed from such molecules are called compounds. Water (H2O), for instance, is a compound consisting of molecules that contain two atoms of hydrogen joined with one atom of oxygen.

Compounds that are part of organisms or that are produced by organisms are known as organic compounds. Today, many organic compounds are produced in laboratories or in large chemical plants. Organic compounds contain carbon in combination with other elements such as hydrogen, oxygen, nitrogen, and sulfur.

There are many kinds of organic compounds. Those that are necessary for life belong to one of four large groups — carbohydrates, lipids (or fats),proteins, or nucleic acids.

Carbohydrates

Carbohydrates contain atoms of only three elements — carbon, hydrogen, and oxygen. They are used in the cell as fuel, and they play an important role in the production of energy. Plants are made up largely of carbohydrates; smaller amounts are found in animals. Some common carbohydrates are sugars, starches, and cellulose.

Carbohydrates are formed from smaller molecules called sugars. Glucose is a simple sugar formed by plants as a result of photosynthesis. The sugar that you find in the sugar bowl at home is called table sugar, or sucrose. It is made up of two simple sugars — glucose and fructose — joined together. Sucrose comes either from sugarcane or from sugar beets. Starches such as cornstarch and potato starch are made up of long chains of glucose units linked together. Another carbohydrate made up of great numbers of glucose units is cellulose. Wood, cotton, hemp, and linen are a few well-known substances that are largely made up of cellulose. The cellulose molecule is even bigger than the starch molecule. While the starch molecule contains about25 glucose units, the cellulose molecule may contain as many as 2,000 units.

Lipids

Lipids, or fats, are made up of carbon, hydrogen, and oxygen atoms, just as carbohydrates are. But in lipids the proportions of each element are different. Usually, lipid molecules contain fewer oxygen atoms than are found in carbohydrate molecules. Lipids are used by cells as a source of energy. They do not dissolve in water, so they are useful in cell membranes for keeping different parts of the cell separate and for keeping the contents of the cell inside the cell.

Some familiar lipids are the solid white fat found on meat; butter, which is the fat from milk; and the oils that come from olives, cotton seeds, and peanuts.

Proteins

Proteins are vital to the structure and function of all organisms. They are also the most abundant of the organic compounds found in living things. Proteins contain carbon, hydrogen, oxygen, and nitrogen. In addition, they may also contain sulfur, phosphorous, and iron. Most protein molecules are very large. One molecule may be made up of thousands of separate atoms. As an example, hemoglobin, the oxygen-carrying part of the red blood cells, is a protein. Each molecule of hemoglobin contains more than 9,000atoms.

Protein molecules are made up of smaller units called amino acids.There are 20 common amino acids. They are called the building blocks of proteins. Living things can construct a great number of different proteins by arranging two or more of these amino acids in various sequences. It is the particular proteins they produce that make one organism differ from another.

Nucleic Acids

Nucleic acids are long, threadlike compounds found in the cells of all living organisms. The two main types of nucleic acid found in living things are DNA (deoxyribonucleic acid), found only in the nucleus of cells, and RNA (ribonucleic acid), found throughout cells. Both DNA and RNA play a central role in passing all traits from one generation to the next.

Chromosomes, which contain the genes that determine traits, are made up largely of DNA. RNA directs the manufacture of the proteins that the cell needs.

The Basic Unit of Life — The Cell

Cells are the basic units of life in all living things. Every organism is made up of one or more cells. While cells show great variety in size, shape, structure, and function, all cells have certain things in common.

Most cells have a nucleus. The nucleus is usually spherical in shape and is found close to the center of the cell. Inside the nucleus are many fine threads or strands. These are the chromosomes. Also in the nucleus are the nucleoli (plural of nucleolus). They play a role in the growth of the cell. The nucleus is the control center for many of the cell's activities.

Surrounding the nucleus is the nuclear membrane. This membrane controls the movement of substances into and out of the nucleus. The material that fills the rest of the cell outside the nucleus is known as cytoplasm. The cytoplasm contains many vital structures, known as organelles. These structures are very small; they are barely visible with an ordinary microscope. They control many different functions of the cell, including energy production, elimination of wastes, and the manufacture of proteins.

The cell membrane surrounds the entire cell. This thin elastic covering controls the passage of substances into and out of the cell. In plants, the cell membrane is surrounded by a thick cell wall made of cellulose.

The cytoplasm in plant cells contains some structures not found in animal cells. These include bodies that contain chlorophyll, a pigment needed for photosynthesis. Also present are vacuoles, which are fluid-filled sacs within the cytoplasm that contain dissolved minerals.

Bacteria have cells that are not like those of other organisms. Bacteria do not have a nucleus enclosed in a membrane. Instead, they have a single molecule of DNA. They are missing many of the structures that are found in the cytoplasm of other cells. Although they have a cell wall in addition to the cell membrane, the wall is not made of cellulose as it is in plants. The bacterial cell may be one of the first kinds of cells to appear on earth.

In a single-celled organism, all of life's functions are carried out by the single cell. The cell takes in food, metabolizes the food, and gets rid of wastes. It reproduces by dividing in two. In a many-celled organism, different life functions are carried out by groups of cells that specialize in carrying out specific duties. A group of identical cells is called a tissue.For example, bone cells, which lay down hard layers of calcium minerals, form bone tissue. Muscle cells, which have the ability to contract, form muscle tissue.

An organ is made up of various tissues arranged to do special work for the body. Each organ, therefore, contains several different kinds of cells. The heart is an organ that pumps blood throughout the body; it is made up largely of muscle tissue. The eye is an organ for seeing; it contains tissue made up of cells that are able to sense light. The eye also contains muscle tissue to allow movement. The lungs contain tissue that is elastic and able to exchange gases easily. The stomach has muscle cells and cells that produce juices to digest food.

A set of organs in the body is grouped into a system. The framework of the body with all its bones forms the skeletal system. The muscles that move and support the skeleton and also move other parts of the body form the muscular system. The digestive system includes such organs as the stomach, liver, and intestines. All together, the systems of the body make up a living being, or an organism.

Living Things and Their Environment

The study of the relationship between living organisms and their environments is called ecology. Ecology is concerned with the biosphere — the part of the earth's air, land, and water that contains living things. It extends about six miles above sea level and as far into the soil as roots and micro-organisms are found. The biosphere includes the surface waters and the ocean depths. Compared to the size of the earth and the immensity of space, the biosphere is just a thin band.

Physical Environment

Conditions in the biosphere are controlled by many different physical factors. One of the most important factors is the movement of the earth around the sun, which causes the seasons. Another is the movement of air and water over the earth's surface. These factors cause a wide range in temperature and rainfall from place to place and at different times of the year. Surface features such as hills, mountains, and deserts also affect the kinds of plant and animal life found in different parts of the world.

>Temperature.

Living things can exist only within a very narrow temperature range, from about 32 to 120°F (0 to 49°C). There are few exceptions to this. Below these temperatures the water in cells freezes and many life processes stop, just as very high temperatures will affect the proteins in cells and stop life processes.

Some animals are able to control the temperature inside their bodies to a certain extent. These animals, known as warm-blooded animals, produce enough heat energy to keep their internal temperatures at a certain level even if the outside temperatures drop to a low level. They do lose heat, though, and must be protected by fur, feathers, or warm clothing if exposed to low temperatures for long periods. Birds and mammals, such as humans, horses, bears, and whales, are warm-blooded animals. Other animals have body temperatures that change as the surrounding temperature changes. They are known as cold-blooded animals. Fish and reptiles, for example, are animals that are cold-blooded.

Temperature is affected by latitude, which is a measure of the distance north or south of the equator. The areas near the equator generally have the warmest temperatures, while the coldest temperatures are found in the areas around the North and South poles.

Temperature is also affected by the height above sea level, or altitude. The higher the altitude, the lower the temperature. Even near the equator the temperature on top of a mountain can be quite low.

Water.

Water is essential for life. Many of the essential processes that take place in cells depend on water. For life to continue, therefore, there must be a plentiful supply of fresh water in the environment. Much of the water on earth is in the world's oceans, lakes, and streams. The rest is either in soil or rock, in the atmosphere, in the bodies of living organisms, or frozen in polar ice and glaciers.

Water is a basic part of every cell in every living organism. About 70percent of the human body is water. Water helps digestion by making it possible for the digestive chemicals to break the food down into molecules that can be used by the body's cells. Water in the blood helps to carry these molecules to all parts of the body. Water washes out wastes that accumulate in the body, and water helps to control body temperature, too, when it is released during the process of perspiration.

Some animals have become well adapted to living in an environment with little or no water. The kangaroo rat is a good example. This desert animal never drinks water. Its entire diet consists of dry seeds and other foods that contain little or no water, yet 65 percent of its body is water.

As the kangaroo rat burns food for energy in its body, a small amount of water is formed, and the animal is able to live on this water. Because it has sweat glands only on the pads of its toes and spends the hottest part of each day in cool burrows beneath the desert sands, it loses almost no water through perspiration.

Atmosphere.

The atmosphere is the blanket of air that surrounds the earth. Air is a mixture of colorless, odorless, and tasteless gases. About four fifths of the air is nitrogen, and about one fifth oxygen. The rest is small amounts of gases such as carbon dioxide.

Most living things need oxygen in order to live. The oxygen is needed in chemical processes that change food into energy. Organisms that live on land take in oxygen from the atmosphere. Land animals have organs such as lungs to make this possible. Organisms that live in water take in oxygen that comes from air dissolved in the water. Fish, for example, have gills that allow them to take in oxygen.

The process of respiration in plants, whether on land or in the water, happens as they take in oxygen and give off carbon dioxide through small openings in their leaves. But unlike other living things, plants also carry on photosynthesis. In this process they take in carbon dioxide and give off oxygen.

There are certain organisms that can live without oxygen. In fact, they are not able to live in the presence of air. These organisms are known as anaerobic bacteria. These tiny microbes usually live in soil, in water, or in places like the mud at the bottom of a pond or stream. A very dangerous kind of anaerobic bacteria can grow in airtight food containers if it is not killed by heat before the containers are sealed. These are the microbes that cause botulism, an often fatal food poisoning. A close relative of this bacterium is the one that causes a very serious infection called tetanus. This organism can grow in places such as deep puncture wounds, where there is no air.

Plants depend on soil for water and minerals, which are taken in through their roots. Soil is a mixture of particles of rock, water, air, tiny living organisms, and dead organic matter. It is formed by the weathering of rock, with the addition of material from organisms that are decaying.

Generally, plants prefer a certain kind of soil in which to grow. One type of plant may need a dry, sandy soil, while another may need a moist soil. For example, pine trees do best in sandy soil, while soil that includes clay, sand, and organic matter is best for forests of beech and maple trees. The wet, swampy soil of bogs is ideal for larch, white cedar, and cypress trees. The shallow, rocky soil on some mountain slopes favors forests of redwood and spruce.

Factors such as average temperature, amount of moisture available, and soil type affect the type of plant life that can grow in an area. A biome is a community of specific types of plants and animals that covers a large area of the earth's surface. Each biome is made up of many habitats — the place where a particular plant or animal normally lives and grows. The type of biome in a region is generally determined by the kind of climate in that region. For example, tropical rain forests can be found in areas of abundant rainfall and hot temperatures. In areas of hot temperatures where rainfall is severely limited, deserts can be found. The tundra is the area found around the Arctic and Antarctic circles and on some high mountaintops. The ground is permanently frozen just a few feet below the surface, and the plant life consists of mosses, lichens, grasses, and shrubs.

Biorhythms.

Living things are also affected by light in many ways. Because the rotation of the earth on its axis causes night and day, organisms are exposed to alternating periods of light and dark. Organisms respond to this in various ways. Some flowers, for instance, open during the day and close at night. Many animals are active during the day and sleep at night, while others do just the reverse. In humans, there is a daily rise and fall in blood pressure, blood sugar level, body temperature, and other variables. These natural cycles are called biological rhythms. They can occur every few seconds, as in the beating of the heart, or within a single day, as in the opening and closing of flowers such as roses and morning glories. Changes that occur on a daily basis are known as circadian rhythms.There are also rhythms that occur on a monthly or yearly basis. Migrations of birds and other animals are examples of yearly cycles, which may be triggered by seasonal changes in the length of daylight and in temperature. All are examples of biorhythms.

It is important for plants and animals to produce offspring at a time of year when conditions are right for the survival of their young. It would be very difficult, for instance, if deer gave birth to fawns just as winter was beginning. A much more favorable time is in the early spring. It is believed that there is a biological clock in animals and plants that helps them respond to external clues such as changes in daylight and temperature.

Biological Environment

Living things are not only affected by the environment in which they live, they are also affected by other living things in their environment. One way to study the effects of living beings on one another is through the study of their groups. The two basic groups are populations and communities.

Populations and Communities.

Most of us are familiar with the word "population." Knowing the population of a city, state, or nation means knowing how many human beings live there. But when biologists use this word, they have a different meaning in mind. They are talking about the number of any one kind of living thing with shared characteristics found in a particular place. As an example, a biologist might report a population of 200 snails of one species in a particular pond. Or, there might be a stand of 17 white birch trees in a particular forest.

A basic factor in determining the size of a population is the environment. In general, if there is plenty of water, good soil, and a suitable climate, the environment can sustain a large population. But if some elements are in short supply, only some individuals will survive. The others have to find another area or face the danger of dying.

A community includes populations of different species in a specific location interacting with each other. Usually a community is a complete system. It can sustain itself without taking in anything it needs from outside the community.

Webs of Life.

Most communities contain one or more food chains. A food chain consists of a series of organisms, each one using the next one in the chain as part of its food supply. In the process, energy is transferred from one part of the chain to the next. Most biologists divide the members of a food chain into four separate groups.

Producers. Green plants are the primary producers of the food chain. They take minerals and water from the soil, carbon dioxide from the air, and energy from the sun to produce materials that make up their cells.

Primary consumers. These are the animals, called herbivores,that feed directly on plants. Cows and grasshoppers are both examples.

Secondary consumers. Organisms that feed on the primary consumers are known as secondary consumers. Wolves eat rabbits and other small animals. Some animals are primary and secondary consumers; bears, for example, eat plant berries and fish.

Decomposers. Decomposers feed on dead organisms. In this manner, the dead organisms are used to keep the food chain going. Bacteria, fungi, and some insects are among the decomposers.

The food chains within a community are often connected and related, one to the other. All together they form a food web, or web of life.

In some instances, there are special relationships between the members of a community. For example, some of the consumers are predators.Predators are animals that kill other animals, which are called prey, for food. Eagles, owls, wolves, and humans are a few well-known predators.

Predators are usually well suited for catching, killing, and eating their prey. They have keen senses of sight, smell, or vision, so they can detect other animals from far away. Many predators can move quickly to catch a fleeing creature. They have powerful claws and jaws to hold and kill their victims.

Some animals are scavengers. Scavengers eat the bodies of dead animals. By eating animals that have died or have been killed, scavengers return the materials of the dead animals to the food cycle. Vultures and jackals are scavengers.

Some organisms of different species in a community live together in special relationships. The general term for such relationships is symbiosis.

One form of symbiosis is commensalism. In commensalism one organism gains and the other seems to neither gain nor suffer. The remora is a small fish that uses a suction disk on top of its head to attach itself to a larger fish, such as a shark. The remora gains by being carried around its environment and by being able to eat scraps of the shark's food. The relationship does not seem to harm the shark in any way.

Another example of commensalism occurs when a woodpecker chisels out a hole in a tree and then abandons the hole. After the woodpecker is gone, a bluebird may make a nest in the hole. The bluebird has gained a home, and the woodpecker has lost nothing.

Mutualism is a type of symbiosis in which both organisms gain from each other. One kind of crocodile opens its mouth wide and allows a small bird, the Egyptian plover, to pick food particles from between its teeth. This helps both organisms. The crocodile gets its teeth cleaned while the plover gets a meal.

Mutualism is also found in the intestines of humans. Certain bacteria living there have a plentiful supply of food and water. In return, these bacteria produce vitamins that are helpful to humans.

Another kind of symbiosis, parasitism, occurs when one organism(the parasite) takes nourishment from another, usually larger, organism (the host) and harms the host in some way. A number of parasites cause human diseases. Some parasitic bacteria that live in or on human beings can cause such diseases as diphtheria and typhoid fever. Athlete's foot and ringworm come from another type of parasite, a fungus.

The opposite of symbiosis is competition. Competition can occur within one species, with individuals fighting each other for food, water, light, or living space. It can also occur between individuals of different species fighting over the same resources.

Deer living in communities near woodlands where humans are building houses must compete with other deer for the dwindling food supply. In many areas, there are too many deer for the amount of food that is available. As a result, large numbers of deer may die of starvation. In general, the ones who win the competition for food and space are those better equipped to find food and to fight off other hungry animals.

There are many examples of competition between species. Both hawks and owls eat mice. Every mouse that a hawk eats is one less for an owl, and, of course, the reverse is also true. Cattails and duckweed plants often grow next to each other and compete for space, light, and an adequate supply of water.

The Kingdoms of Living Things

The Greek philosopher Aristotle (384-322 BC) attempted to classify living things. He named two large groups: plants and animals. Plants were further divided into those with soft stems, those with one hard, woody stem(trees), and those with more than one woody stem (shrubs). The animal group was further divided into those that live in water, those that live on land, and those that live in the air. For many centuries Aristotle's system was accepted without question. It was not until the middle of the 1700's that a Swedish botanist and physician, Carolus Linnaeus (1707-78), introduced a new system that has become the basis for modern biological classification.

In the Linnaean system, the species is the basic unit of classification. Each organism is identified by two names. The first name is the genus, which is a group made up of similar species. The second name is the species, which is a more particular group of similar individuals. Members of one species can mate with other members of the same species but not with members of different species. The species name identifies the exact type of organism within the large group of the genus. For example, dogs, wolves, and coyotes all belong to the same genus, Canis. To make a distinction between these similar species, dogs are named Canis familiaris; wolves are named Canis lupus; and coyotes are named Canis latrans.

Using the genus and species name avoids much confusion. For example, the American mountain lion has close to 20 common names — among them are puma, cougar, panther, silver lion, king cat, and varmint. Using the scientific name, Felis concolor, is one way to make sure that people in different parts of the country are talking about the same animal.

The scientific name also helps to describe the organism and show its relationship to other organisms. For example, when they are given the scientific name of an organism with which they are not familiar, biologists will be able to tell you something about the organism's body structure, how it reproduces, what foods it eats, and other basic characteristics.

Species joined with similar or related species form the genus. The genus, too, can be joined with other similar genera (plural of genus) to form a larger group called a family. An example is the Apoidea, or bee, family. Among the genera in the family Apoidea are the honeybee (genus Apis),the bumblebee (genus Bombus), and the mason bee (genus Chalicodoma).

Families that have features in common can be grounded together to make an order. The Diptera order includes all the various families of flies and similar insects. There are families of fruit flies (Trypetidae),horseflies (Tabanidae), daddy longlegs (Tipulidae), and mosquitoes (Culicidae)all within the Diptera order.

Groups of related orders make up a class, the next higher level of organization. The largest class of plants, Angiospermae, includes more than 280,000 species of flowering plants. These plants are grouped into more than 50 different orders. Among the larger orders are Ranales (magnolias and buttercups), Rosales (roses), Geraniales (geraniums), and Umbellales (carrots).

Going one step further, related classes are combined to form either a phylum (for animals) or a division (for plants and plantlike organisms). As an example, the phylum Porifera contains all the sponges. There are more than 5,000 species. Most of them fall into three classes: Calcispongiae(shallow water), Hyalspongiae (deepwater), and Demospongiae (brightly colored).

The highest level of organization is the kingdom. Originally, taxonomists (scientists whose specialty is classification) identified only two kingdoms — animal (animalia) and plant (plantae). It became necessary to add another kingdom, protist (protista), when more was learned about microscopic living things. They did not fit very well into either the animal or plant kingdoms.

As knowledge of the world of living things increased because of better laboratory instruments and advanced research, two more kingdoms were added — monera and fungi. Here are the five kingdoms, their characteristics, and examples:

Kingdom Monera

One-celled, or form groups; cells lack organelles found in other kingdoms. Includes bacteria, such as cyanobacteria.

Kingdom Protista

Usually one-celled, with organelles. Some carry on photosynthesis; some move by cilia, flagella, or a flowing movement. Includes amoebas, paramecia, diatoms, and dinoflagellates.

Kingdom Fungi

Mostly many-celled; food obtained by absorbing material from dead organisms; cell walls made of chitin (carbohydrate found in insects). Includes mushrooms, molds, mildews, yeasts, water molds, and slime molds.

Kingdom Plantae

Many-celled with organelles; tissues; organs; carry on photosynthesis; cell walls made of cellulose. Includes mosses, ferns, conifers, and flowering plants.

Early biologists also divided life into two main branches, the prokaryotes and the eukaryotes. The prokaryotes included bacteria, which lack a nucleus; the eukaryotes included animals, plants, fungi, and protists, which have a nucleus in their cells.

However, in the late 1970's, scientists began discovering microscopic organisms that did not fall easily into either branch. Many of these organisms, now called archaea, live in harsh environments, such as hot springs and sulfur-spewing vents on the ocean floor. Later studies showed that archaea, which look like bacteria, are a separate branch that is more closely related to eukaryotes in cell structure.

There are more than 1.5 million known organisms. Taxonomists attempt to classify these organisms to show their relationships to each other and possibly the way in which they have evolved. This is a difficult task and scientists do not always agree on the best method of classification. Comparing certain proteins of two different organisms has helped in some cases to determine how closely related the two organisms are.

The Origin of Life

No one knows exactly how or when life first appeared on earth. Most scientists guess that it was about 4 billion years ago. Scientists also do not know what the first living organisms looked like, although fossils of bacteria-like cells about 3.5 billion years old have been found. Over the billions of years since then, their descendants have evolved and produced organisms the size of dinosaurs and whales.

Theories of the Origin of LifeReligious Explanations.

Some people do not accept the view that life on earth had such primitive beginnings. These people, called creationists, believe that the story of creation in the Bible is a literal explanation of the origin of life. In their view, God created all the living plants and animals as we now know them. Most scientists today, however, feel that the Bible is not a literal or a scientific report.

Spontaneous Generation.

For centuries, people believed that living things could arise from nonliving things. It was thought, for instance, that frogs were born from mud and that rotting meat gave rise to flies. This theory is known as spontaneous generation.

Italian physician and scientist Francesco Redi (1626-97) was one of the scientists of his time who did not accept the idea of spontaneous generation. In 1668, he performed an experiment that showed what actually happens. Redi placed pieces of meat in flasks with wide mouths. He left some of the flasks uncovered. The rest of the flasks were covered with cloth that had an open weave. Within a few days all of the meat began to decay. Flies, attracted by the smell, landed on the meat in the open flasks and laid eggs. Soon thereafter, the eggs hatched and small wormlike maggots hatched from the eggs and began crawling around the meat. Flies were also attracted by the smell from the other flasks. But, unable to reach the meat because of the cloth covering, they laid their eggs on the cloth. When these eggs hatched, the cloth, not the meat, was covered with maggots.

Redi's work showed that the flies on rotting meat came only from other flies. It was more than 200 years later, though, that experiments by scientists such as Louis Pasteur (1822-95) finally convinced people that living things did not arise spontaneously from nonliving things.

Panspermia.

Among other theories on the origin of life is the so-called germ theory, or panspermia. According to this theory, there are germs spread throughout the universe. These germs grow when they reach a place with favorable conditions such as the earth's. Panspermia maintains that the organisms we know of sprang from such germs. Few scientists today believe in this theory.

Modern Theories.

In 1953, Stanley Miller, a young scientist, tried an experiment in which he re-created conditions thought to exist on earth before life was present. About 4 billion years ago the earth's atmosphere might have contained the gases methane, ammonia, hydrogen, and water vapor. The primitive seas would have been very hot; the atmosphere turbulent with violent lightning storms.

For his experiment, Miller placed hot water and the four gases found in the ancient atmosphere in a flask. Then he sent lightning like sparks through the mixture of gases and water.

After a week, Miller found that the water had turned red and that two amino acids were present in it. As you know, amino acids are basic units of proteins, and proteins are the building blocks of all living things.

The steps between the formation of simple amino acids and the appearance of the first cell, however, are not at all clear. There are many theories but no real answers to the question of how life on earth began.

The theory demonstrated by Miller's experiment proposes that energy from bolts of lightning might have formed molecules of amino acids from the water and gases in the earth's atmosphere. In time, the seas were thick with amino acids. As the amino acids floated around, they kept bumping into one another. From time to time, some stuck together, forming large molecules. Eventually molecules that contained hundreds of these amino acids were formed. Then, by chance, different larger molecules formed. When these molecules bumped into certain smaller molecules, the smaller ones were added to the larger ones.

These big molecules were special in another way. They grew to a certain size and then divided. Instead of being one molecule, they became two identical molecules. Each new one went on taking in smaller molecules, growing, and then dividing again. In this way, complex molecules that eventually gave rise to DNA and RNA may have been formed.

Life on Other Planets

For many years humans have wondered whether life exists elsewhere in the universe. Many scientists today believe that there is a good chance of finding life on other planets.

Where Do Scientists Look for Life?

For life as we know it to exist on another planet, the conditions would have to be suitable. The planet must be the right distance from its sun, so that the temperature is not too hot or too cold. There must be plenty of water and enough oxygen in the air to sustain life. In addition, the atmosphere must be thick enough to keep out ultraviolet rays and other dangerous radiation from space. Of course, there may be other kinds of living things in the universe that demand entirely different conditions

How Do Scientists Look for Life?

Over the years different scientists have suggested ways to make contact with creatures on other planets. In 1959, a research effort known as SETI, or Search for ExtraTerrestrial Intelligence, was established. From its work came the abbreviation ET, for an extraterrestrial — a creature living on another planet.

The tool used by SETI researchers is the radio telescope — a huge antenna, shaped like a metal dish up to 1,000 feet (300 meters) across.

Although they expect to hear signals from natural sources, SETI scientists are searching for special signals — ones with a form or pattern different from ordinary radio waves. Over the years, scientists have picked up some strange signals on their radio telescopes. Each time, though, they were able to explain them as resulting from natural causes.

Scientists also are trying to send messages to other life forms that might exist in the universe. In 1972, the Pioneer 10 space probe was launched from the Kennedy Space Center in Florida. It carried drawings of a nude man and woman, the Pioneer spacecraft and its path through the planets, the position of the sun in the galaxy, and a diagram of the hydrogen atom are inscribed on the plaque. Perhaps some ET will spot Pioneer 10 during its long journey through space, will understand at least some of the drawings, and will find a way to contact us.

In 1974, scientists beamed a powerful signal into space as another way of making contact with ET's. Using the same binary system as computers, the message included the numbers from 1 to 10, the atomic numbers for several elements, the formula for DNA, a diagram of the structure of a DNA molecule, and the human population on earth.

Thus far, there is no direct evidence that ET's have sent us messages or received messages from us. But the scientists working for SETI are not discouraged. They think that we will be in touch with intelligent beings on other planets some day.